CN1186558A - Optically diffractive structure - Google Patents

Abstract

The invention relates to a structural arrangement consisting of one or more surface regions having optically diffractive structures, especially for visually identifiable security devices for valuable documents, e.g. bank notes, credit cards, identity papers or cheques or similar objects to be protected. To make it even more difficult to forge and especially copy such a structural arrangement, the invention proposes to form the structure in such a way that, to generate certain optical information in certain viewing directions, there are partial regions (10, 12, 14, 32, 34) in one or more of the surface areas (8, 30) with an identical structure (16, 18, 20; 36, 38) as far as the structure parameter of the optical depth, and that the optical depth of the structure (16, 20; 36) is constant over the extent of a partial region (10, 14; 32) but different from the optical depth of the structure (18; 38) of another partial region (12; 34).

Description

Structures with optical diffraction effects

The invention relates to structures comprising a surface area comprising one or more structures having an optical diffractive effect, in particular for the visual identification of documents of value, such as banknotes, credit cards, passbooks or check documents, or other objects requiring security optical security elements.

When this structure is employed, visually perceptible information is transmitted to the examiner through diffraction and/or refraction of incident ambient light. Also, with the proper equipment, machines can detect this type of optical information. In the simplest case a structure of this type is provided by a particular rectilinear wavy relief structure on the surface of the surface region of the carrier and where incident ambient light is reflected with simultaneous diffraction and/or or refraction. The term wave or relief structure in this context is not restricted to structures having a continuous surface profile over the cross-section of the surface area, in particular sinusoidal waves, but may also include rectangular, stepped or wedge-shaped surface structures. These surface structures can be periodic or aperiodic in shape. It is likewise conceivable that structures with optically diffractive effects are not formed solely from relief structures, but that changes in the refractive index in the form of the structure can also provide structures with optically diffractive effects.

The diffraction of incident light or light passing through the structure at the structure of the surface region and the information emitted therefrom in the form of an optical diffraction image are determined by the grating or structure parameters. In the case of a relief structure, the structural parameters refer to the number of waves or grating lines per unit length of the surface area, the so-called spatial frequency, as well as the orientation and cross-sectional shape of the relief structure. The shape of the cross-section is determined by the difference in height in the relief structure, more specifically by the height difference between the individual raised portions of the relief structure relative to each other and the difference between the raised portions of the relief structure and the recesses or depressions. The height difference between them is determined in two ways. In case the structure is not formed by a relief structure but by changes in the refractive index arranged in a structured manner, the structure parameters are determined according to the foregoing. Furthermore, the refractive index of the optically active layer or layers of optically active layers must also be taken into account. By means of an appropriate shape and layout of the structure, a structure can be obtained. The structure affects the phase relationship of the incident light in such a way that a given optical information content can be emitted in a given range of viewing angles, and thus perceived by the observer, and emitted in another range of viewing angles. information. The change in the phase relationship dependent on the structure is equal to the product of the refractive index and the geometrical wavelength in or on the structure. In the case where the wave is diffracted or reflected at position x ₁ (e.g. a raised portion of the structure) and at position x ₂ (e.g. a depression in the structure), the optical phase difference (OPD) is: OPD(x ₁ , x ₂ )＝∫n(x ₁ ，z)dz-∫n(x ₂ ，z)dz.

From this relationship it follows that whether the reflective relief grating is varnished, as is currently practiced, or not, there will be a difference because the difference is not only a difference in height, but also Also relative to the difference in refractive index of the coating layer, this is relevant. Visually perceivable information corresponding to the structure of the surface area, depending on the illumination or viewing angle, in particular information on the reliability of the protection content, can be transmitted to the inspector in the form of reflected light or light transmitted through the structure .

By using a security element known per se with a structure having an optical diffractive effect, for the objects to be protected more securely mentioned at the beginning of the description, the content of the information about the reliability of the protected object is even useful to an inexperienced outsider It is also clearly visible. At the same time, counterfeiting, for example in the form of reproduction, is impossible or very difficult in view of known counterfeiting methods, in particular optical reproduction methods.

For example structures are known in which by means of a certain variable among the aforementioned structural parameters - spatial frequency, orientation and cross-sectional shape of the relief structures, differences in height or phase in the relief structures - the examiner can A given visually perceivable optical information content produced from a surface area observed within a certain range of illumination directions and a given range of viewing angles, but not perceived from another surface of a structure within the same range of viewing angles Other optical information content generated by the region. Rotation of the structure-laden carrier about an axis in the carrier plane or an axis perpendicular to the carrier plane changes the information arising from the first observed surface area - in particular this surface area may appear black - The other surface region, which initially appears black, emits optical information, for example in the form of a color print.

The object of the invention is to make counterfeiting, in particular copying, structures of the above-mentioned type more difficult, in particular to increase the variety of coding options for optical information perceivable over a range of viewing angles.

According to the invention, the aforementioned object is achieved in that, in order to produce a given optical information content in a given direction within one or more surface areas of the structure, sub-areas are provided on said surface areas, each sub-area having an optical depth Structures that are the same except for different structural parameters, and the optical depth of the structure is constant within one sub-region, but different from the optical depth of the structure in another sub-region.

In the case of a pure relief structure, the optical depth depends on the geometric depth of the structure; this optical depth corresponds to the optical wavelength difference between two light beams reflected at a raised portion or a depression, respectively, of the relief structure. In the case of structures with locally varying refractive indices, the optical depth, which determines the phase relationship of the light diffracted at the structure, is given by the different refractive indices as well as the different thicknesses of the coating. The optical depth of a structure determines the amount of light lost by diffraction from the geometric reflection direction, ie the diffraction index of the structure. Two structures that are identical except for the optical depth parameter differ from each other in the content of optical information conveyed in a given viewing direction. A combination according to the invention of a structure with a given first optical depth and the required wave optics information content in a given range of viewing angles, and at least one or more structures with a second or more optical depth and another The desired combination of information content provides further control or encoding options over the structure-related image impressions to be delivered. Different optical information contents can be conveyed from the same region of the structure if, for example, subregions are provided which have an otherwise identical structure except for the optical depth, and the size of this optical depth cannot be resolved by the naked eye.

Like this, by means of the layout corresponding to the required image impression of the sub-regions with the structure of the invention, the image subject or image element produced by the structure of the first light depth can be in the first viewing direction in the first color In the other viewing direction, the image motifs or image elements produced by the structured subfields of the second optical depth appear in a different color. In order to achieve such an effect, it has been found to be advantageous, for example, to relate the light depth of the individual structures to a given visually perceivable wavelength or wavelength range. The optical depth of a structure can also be related to a wavelength or range of wavelengths that can be detected by a robotic device. In general, the diffraction rate of a usual rectangular or sinusoidal grating is maximized when the optical depth results in a phase delay of π. Since the fluctuations are periodic, for these symmetric gratings with optical depths of phase delay (2K-1)π, K=1,...N, the maximum diffraction rate repeats to a first approximation. Geometrically for a reflective grating of the first order (K=1), the depth is calculated by the formula δ _z =R/4n, where n denotes the refractive index of the cladding. In the case of a sawtooth infrared grating, the maximum diffraction rate is obtained at a depth corresponding to a phase delay of 2Kπ. Higher level gratings (K > 1) are K times deeper and thus more difficult to produce. Optimum controllability is obtained for each structure whose optical depth lies in the range from 0 to several times, preferably up to 10 times, the wavelength considered.

In particular, when a uniformly appearing image subject or element is desired, it is advisable to provide a group of subfields which preferably have the same structure, which can no longer be distinguished by the naked eye.

In a development of particular interest to the invention, subfields of structures are provided which comprise structures whose parameters are identical except for the optical depth parameter and which are shifted relative to each other by a fraction of the grating period. In this case, this movement can be achieved by displacing one structure relative to the other within the plane of the carrier. It is also possible to arrange the structures in a direction perpendicular to the plane of the carrier, ie at levels of different heights. This displacement is the same as superimposing the structure with another structure which has an optical diffractive effect and whose scattering direction is especially perpendicular to the structure. This provides a possible way to prevent holographic duplication. Holograms can often be simply copied from a hologram. Mainly, argon-ion or helium-cadmium lasers are used as holographic lasers because these lasers have strong spectral lines in the blue-green (488nm), blue (454, 442nm) and ultraviolet (about 350nm) frequency ranges. Most photovarnishes suitable for relief on holographic surfaces are sensitive in these ranges. On the contrary, these light-sensitive varnishes are hardly sensitive to light in the red light frequency range. Holograms are recognized to be produced and reproduced with red lasers (e.g. helium-neon lasers); currently, silver-containing gelatin emulsions (photographic negatives) are used for this purpose, but silver-containing gelatin emulsions do not produce suitable surfaces for electroforming or embossing embossed. If the optical depth of the structure is such that the diffraction rate of blue light is very low and red light is very high, then the replication of the structure is difficult when using a blue laser. It is admittedly possible to replicate a hologram onto a red sensitive photographic emulsion and subsequently onto a blue sensitive relief material, but has disadvantages well known to those skilled in the art and is very costly.

The extinction of the optical information emitted in a given viewing direction can be achieved by means of additional diffraction phenomena due to the movement of the structure of one sub-region relative to the structure of another sub-region, so that by means of the shape of the aforementioned sub-regions, Effectively prevent copying. Thus, in one structure, a sub-area, such as one-half of a pixel, may contain such an optical depth that the diffraction rate of blue light is minimal, which means that at the same time, the diffraction rate of red light, although not maximum, is It still needs to be considered (for the blue light of R=442nm of the helium-cadmium laser, the optical depth of the minimum diffraction rate of the symmetrical rectangular or sinusoidal waveform grating is exactly 442nm, and the diffraction rate of the red light with a wavelength of about 600nm is not largest, but its size is still worth considering). Another sub-region, such as the other half of the pixel, has the maximum diffraction rate of red light at the light depth, that is, for example, 300 nm. In the appropriate viewing direction, sub-regions or pixels may be perceived as a medium-light blue light. Now the structure of the second sub-region can be shifted by a fraction of the grating period (approximately 2π/3) relative to the structure of the first sub-region so that the part of the incident light that is diffracted from the sub-region is substantially eliminated in the In red light, black writing can thus be produced, for example, on a red light background. When such a hologram is reproduced with blue laser light, only blue laser light of a specific wavelength, then it is not possible to reproduce the subregion whose optical depth of the structure corresponds exactly to the wavelength of blue light, for which the diffraction rate is lowest. Structures with such an optical depth cannot be "seen" by blue laser light and thus cannot be replicated. The other sub-regions which have a maximum diffraction rate for red light also have a higher diffraction rate for blue light, so that the structure of these sub-regions can be reproduced satisfactorily. If a structure replicated in this way with blue light is "read out" with red light, in which the subtractive color effect disappears and the pattern appears moderately reddish at that location. Due to poor contrast under red light, the above-mentioned inscriptions are difficult or impossible to read, making it possible to distinguish genuine structures from reproductions.

In order to produce a homogeneous image impression, it is advisable to arrange sub-fields of the above-mentioned type adjacent to one another, in particular to provide a number of sub-fields which cannot be discerned by the naked eye.

It is to be pointed out that the structures according to the invention do not have to be single-layer reliefs (varnish reliefs), but can also be multi-layer systems, so that a structure can also be printed on a multi-layer substrate comprising varnish layers with different refractive indices middle. In this case, the different varnish layers absorb differently in addition to having different refractive indices. Such multilayer systems can lead to further interference effects and cannot be reproduced with simple holographic methods.

Specific details, features and advantages of the invention will be apparent from the accompanying drawings and the following description of a preferred embodiment of a structure according to the invention.

FIG. 1 shows a security element of a document of value having a structure consisting of a number of surface areas represented diagrammatically.

Figure 2 is a diagrammatic cross-sectional view of a structure according to the invention.

Figure 3 shows two sub-regions of a structure according to the invention with mutually shifted relief structures.

FIG. 1 shows a certificate of value carrier 2 with a security element 4 comprising a structure in which a visually perceptible information content is stored or programmed in the form of an image 6 . The security element 4 or structure comprises a plurality of diagrammatically represented surface regions 8 in which one or more relief structures which cannot be represented in FIG. 1 are present.

Figure 2 shows a cross-sectional view of a part of a structure according to the invention. FIG. 2 shows in particular subregions 10 , 12 , 14 which may belong to the same surface area of the structure. Each sub-region 10 , 12 , 14 includes a respective rectangular relief structure 16 , 18 , 20 . The relief structures 16, 18, 20 contain the same spatial frequency, the same ratio of signal pulses to space pulses, and the same geometry, they differ only in terms of geometric depth. The relief structure 16 , 18 , 20 is constant over the respective subregion 10 , 12 , 14 .

The depth of the respective relief structure 16, 18, 20 is linked, for example, to the desired image impression in a given direction or to a given color or color impression. According to the information stated at the beginning of this specification, for the rectangular relief structure illustrated here, this depth is equal to 1/4 of the wavelength of maximum diffraction index (or an odd multiple of this 1/4 wavelength), so that in the A phase difference of π can be generated between the waves reflected by the convex part and the concave part of the structure (if the structure is coated with a transparent coating, the refractive index of the coating also needs to be considered). Thus incident light at an angle of incidence a can be seen in the form of a color print of the sub-regions 10, 14 or their identical relief structures 16, 20 when viewed in the viewing direction β. In comparison, with a suitably determined light depth, the relief structure 18 of the subfield 12 can deliver another color print. If, for security purposes, e.g. when checking authenticity information content with a machine device, light of a given wavelength is irradiated on the security element or a structure thereon at a given angle of incidence, then a sub-area of a given optical depth is illuminated by the incident light In a given viewing direction, the color is displayed, while for this wavelength, the diffraction rate of other sub-regions or relief structures is so small that these sub-regions appear black.

The surface regions can now be formed in such a way that the relief structures, which have the same except for the optical depth parameter, are shifted relative to each other by a fraction of the grating period, as schematically shown in FIG. 3 . Reference number 30 highlights a surface area of the structure, for example a pixel of the security element. The surface region 30 comprises two subregions 32 , 34 each having a respective relief structure 36 , 38 . The relief structures 36, 38 are identical except for the optical depth parameter, ie they contain the same spatial frequency, the same cross-sectional shape, and the same ratio of signal pulses to space pulses. The depths of the relief structures 36 and 38 are each chosen such that the diffraction rate is maximized for mutually different wavelengths. Likewise, the relief structures 36 , 38 are displaced relative to one another by a fraction of the grating period g within the carrier surface of the security element. When looking at the surface region 30 what the eye sees is a superposition of the wavefields emanating from the sub-regions 32,34. Mathematically, this superposition can be described as the numerical square of the amplitudes diffracted in the sub-regions 32, 34 with a relative value of 1 or Exp(iΦ), where the phase Φ is given by 2πδx/g, then the intensity is as follows:

I＝(1+Exp(iΦ)·(1+Exp(-iΦ))＝2+2cos.Φ.

The brightness of the surface area can thus again be adjusted by a relative movement or displacement of the relief structures 36 , 38 relative to one another. It can be seen from the foregoing principles that the π phase shift corresponds to moving the grating by 1/2 the grating period, so that, for example, the elimination of the first diffraction order can be achieved by means of the beam splitting action of the structure.

Claims (8)

1. A structure comprising a surface area comprising one or more structures having an optical diffractive effect, in particular for the visual visibility of documents of value, such as banknotes, credit cards, passbooks or check documents, or other items requiring security An identified optical security element, characterized in that in order to generate a given optical information content in one or more surface areas (8, 30) of a structure in a given viewing direction, a The subregion (10, 12, 14, 32, 34) of the structure (16, 18, 20; 36, 38) with the same structure parameters, and the optical depth of the structure (16, 20; 36) is in the subregion ( 10, 14; 32), but different from the optical depth of the structure (18; 38) of the other sub-region (12; 34). 2. A structure as claimed in claim 1, characterized in that the optical depth of the individual structures is associated with a given visually perceptible or machine detectable wavelength or wavelength range. 3. A structure as claimed in claim 1, characterized in that the optical depth of the individual structures of the sub-regions is in the range from 0 to 10 times the wavelength under consideration. 4. A structure as claimed in one of the preceding claims, characterized in that a group of subfields (10, 14) has the same structure (16, 20). 5. A structure as claimed in one of the preceding claims, characterized in that the surface area (30) or subregions (32, 34) of the surface area (30) have all the same except for the optical depth parameter, and The structures (36, 38) are shifted relative to each other by a fraction of the grating period. 6. A structure as claimed in one of the preceding claims, characterized in that the surface area (8, 30) or a subregion (10, 12, 14, 32, 34) of said surface area (8, 30) are arranged adjacent to each other. 7. A structure as claimed in claim 5 and claim 6, characterized in that the mutually displaced relief structures (36, 38) have such properties that one is continuously blended into the other. 8. A structure as claimed in any one of the preceding claims, characterized in that it comprises a multilayer relief system.